No, you don't. Flat pen doesn't contribute nearly as much to your damage output as flat AP except on a select few champions, like Ryze for example. You can probably search in the Theorycrafting subforum if you're interested in the math. That's not really my forte when it comes to League theory.

If penetration wasn't better than Attack damage or Ability power, then why would anyone get it AT ALL? Isn't that kind of the entire point, that penetration is better? Else why get the reds at all or any penetration?

No, you don't. Flat pen doesn't contribute nearly as much to your damage output as flat AP except on a select few champions, like Ryze for example. You can probably search in the Theorycrafting subforum if you're interested in the math. That's not really my forte when it comes to League theory.

Armor penetration only increases the effectiveness of physical damage, so it's not very useful on mages like Ryze, who deal mostly magic damage. Armor penetration runes (more specifically, Greater Mark of Lethality) are used on some assassins like Kayn and Zed who deal physical damage with their spells.